Achieving early catalyst light-off is crucial if stringent emissions standards are to be met; if light-off is late, the emissions limit could be exceeded even before the catalyst starts to work. This paper presents a detailed simulation study of the factors affecting the light-off of a TWC.
Simulation is not just faster and cheaper than vehicle testing, it also enables more insight into the factors affecting catalyst performance to be obtained. For example, changing the substrate (cell density and wall thickness) affects the rates of heat and mass transport, as well as the thermal mass of the catalyst. In a vehicle test, all three factors are changed at once, but with a simulation each of these factors can implemented one at time to enable the relative importance of these factors to be determined. Thus, it can be shown that while a 1200/2 substrate offers faster heat and mass transport than a 900/2, this is largely offset by the increased thermal mass, so that both substrates give similar light-off performance. Once the catalyst has warmed up, mass transport is the only factor of real importance.
Other catalyst factors considered in this study include catalyst activity, washcoat thermal mass and thickness, heat losses to the surroundings, catalyst volume and catalyst poisoning. Simulation can also be used to investigate the impact of engine calibration, something which is much easier to do by simulation than by modifying the engine calibration. In this case, the effect of exhaust temperature and stoichiometry were investigated.